According to the conventional art, a single crystal tungsten cathode is the only product that has been put into practical use as a field emission type cathode. If a field emission type electron gun using the single crystal tungsten cathode is used for two through twelve hours, the stability of emission current is affected by geometric changes on the cathode surface and the adsorption of gas to the surface of the cathode. This requires the step of heating and flushing on a periodic basis wherein the tungsten cathode is heated by current application. Flushing melts the surface of the cathode, and corrects the structures roughened by ion bombardment. Further, the atom adsorbed on the cathode surface is desorbed, whereby emission current stability is ensured.
Removal of adsorbed atoms from the cathode surface can also be achieved by field evaporation. This method is based on the fact that application of high field to the tip of the cathode causes removal of the adsorbed atom that takes place when the interaction between the field and the electric dipole of the adsorbed atom has exceeded the force of adsorbing the atom onto the atom. The Official Gazette of Japanese Patent Laid-open Sho 52 (1977)-127060 discloses a field evaporation method for cleaning the surface of the field emission type cathode made up of carbon material, carbide material or boride material. In this case, the cathode surface is not smoothed, and need not be smoothed.